7-Keto-3-methoxy-4-hydroxy-5,6-dihydromorphinan was found to exist in an equilibrium mixture with its tautomer, 7-keto-6-deoxydihydrocodeine. An X-ray crystallographic study showed that the molecule was in a space group seen only twice before with organic compounds. It existed, in the solid state, in the form of a double helixan interlacing pair of helices joined by hydrogen bonds. Structures for the compounds involved in the tautomeric equilibrium were deduced from a carbon-13 NMR study, proton NMR, and other spectroscopic techniques.